Some of the servers that provide service to a large number of clients over a network are required to have a particularly high reliability for the nature of the services. A measure often employed to meet such a requirement is to duplicate the server by using another server (so called mirror server) which takes over the service task of the server in case of server failure. In such duplicated server systems, the data (herein referred to as application data) used in application processing on the two servers that make up a server system is always kept identical by connecting the two servers by means of high-speed communication means.
Even in such duplicated server systems, however, if there is a time difference in making the contents of the application data on both servers identical, the service being carried out must be started again from the beginning when the mirror server (secondary server) takes over the tasks in case of server failure while the server that normally provides service for other computers (referred to as the primary server) is actually performing service.
From this point of view, it is desired that the service being carried out at the occurrence of server failure is also continued by seamless transition such that the occurrence of failure is not perceived by the client who requested the service.
The object of the present invention is therefore to provide a method of duplicating servers that makes seamless transition of the service being carried out at the occurrence of server failure possible, and duplicated server system and database system mirror-backed-up according to the method.